Conventionally, a grid moving mechanism for a radiographic/fluoroscopic bed (e.g., patent reference 1: Japanese Patent Laid-Open No. 61-220631), a casetteless fluoroscopic/radiographic apparatus (e.g., patent reference 2: Japanese Patent Laid-Open No. 2-191936), and a fluoroscopic/radiographic apparatus (e.g., patent reference 3: Japanese Patent Laid-Open No. 3-53238) are known as techniques using different grids in fluoroscopy (moving image formation) and radiography (still image formation) in a fluoroscopic/radiographic apparatus.
Patent reference 1 discloses a technique related to a reciprocating grid system, that is, a grid moving mechanism for a fluoroscopic/radiographic bed. This system is characterized by comprising a reciprocal reciprocating mechanism capable of transmitting a reciprocation force to a grid incorporated in a snapshot apparatus which executes X-ray imaging, a retracting mechanism capable of retraction driving and loading driving of the grid with respect to the X-ray imaging area, and a connection mechanism capable of connection and disconnection between the grid and the reciprocal reciprocating mechanism. In loading the grid, the connection mechanism connects the grid to the reciprocal reciprocating mechanism. In retracting the grid, the connection mechanism disconnects the grid from the reciprocal reciprocating mechanism.
According to the description in patent reference 1, when divided imaging on one film is to be executed, continuous imaging (rapid imaging) can be performed. In this case, however, when the grid is reciprocated by using the elastic force of a spring, as in a conventional grid mechanism, the reciprocating motion of the grid gradually attenuates with time. For this reason, stripes often remain on the dividedly taken photo in continuous imaging. The stripes cannot completely be removed. Patent reference 1 discloses a grid moving mechanism for a fluoroscopic/radiographic bed, which can prevent any stripe formation on a divisionally taken photo in continuous imaging and retract the grid from the X-ray imaging area when the grid is not used.
Patent reference 2 discloses a casetteless fluoroscopic/radiographic apparatus. This apparatus is characterized by comprising a grid which removes scattering X-rays from an object that is exposed to X-rays from an X-ray source, a detector which is inserted between the grid and a film to detect the X-rays which have passed through the object and output a detection signal to an X-ray controller which controls the X-ray dose, a contact plate which has, on the lower surface, a lead plate that removes backscattering X-rays and with which the film comes into tight contact, a fluoroscopic grid which is arranged in front of an image intensifier that converts the X-rays into an optical image, and removes the scattering X-rays, a mechanism which opens/closes the contact plate to bring the film into tight contact with it, a mechanism which conveys the contact plate brought into contact with the film by the above mechanism from a film transfer position to a radiographic position, fluoroscopic position, and park position and after the end of fluoroscopic/radiographic, returns the contact plate to the film transfer position, and a control mechanism which changes the contact plate to an adapted grid and detector in radiographic and changes the grid to an adapted fluoroscopic grid in fluoroscopic imaging.
According to the description of patent reference 2, to stabilize the density in imaging, the contact plate that is inserted in imaging integrally includes the grid, the film, and the detector (phototimer) to stabilize the density. Patent reference 2 also discloses changing the grid between fluoroscopy and radiography and changing the grid depending on imaging conditions such as the tube voltage even during fluoroscopy.
Patent reference 3 discloses a fluoroscopic/radiographic apparatus which can set a fluoroscopic mode and a radiographic mode. In a steady state, an image intensifier and a fluoroscopic grid are fixed in the X-ray irradiation field. When the radiographic mode is set, an X-ray film contact holder and a radiographic grid are inserted and arranged in the X-ray irradiation field, and the radiographic grid is reciprocated. This apparatus is characterized by comprising a grid fixing frame which fixes the radiographic grid, a grid reciprocating rail which can move the grid fixing frame between a standby position and an imaging position and reciprocate the frame at the imaging position during movement, a holder driving frame to move the X-ray film contact holder between the standby position and the imaging position, and a driving mechanism which moves the holder driving frame to move the X-ray film contact holder. A driven mechanism is arranged between the grid fixing frame and the holder driving frame to make the grid fixing frame follow the holder driving frame and move along the grid reciprocating rail between the standby position and the imaging position when the driving mechanism is driven to move the holder driving frame between the standby position and the imaging position.
The technique described in patent reference 3 aims at making the radiographic grid reciprocating mechanism cooperate with the imaging frame (a structure made by bringing a film and an intensifying screen into tight contact) inserting mechanism. In the embodiment, the fluoroscopic grid is fixed on the overall surface of a fluoroscopic sensor (I.I.)
Under the above-described technical circumstances, a radiographic apparatus can be considered, which selectively uses different grids between fluoroscopy and radiography by using a single FPD (Flat Panel Detector) sensor using a semiconductor.
However, the radiographic apparatus which executes both fluoroscopy and radiography by using the FPD sensor using a semiconductor has the following problems.    (1) The imaging dose in fluoroscopy is about 1/100 of that in radiography, which is, very small.    (2) In addition to the X-ray dose, the tube voltage also changes between fluoroscopy and radiography. Hence, the grid must appropriately be switched.    (3) When a stationary grid is inserted, moiré is generated in the image depending on the grid frequency and the sampling pitch of the FPD sensor.    (4) If the grid should be reciprocated, the mechanism becomes complex. Hence, setting for obtaining suitable relationships between the image reception frequency and the grid reciprocation period and between the imaging time and the reciprocation speed is also complicated.